In my opinion, the absolute BEST place to see the classic Disney characters, is at Epcot.

Epcot has a thing called the "Character Spot", just past the globe. This is the place where you can meet Mickey, Minnie, Goofy, Donald and Pluto in about 20 minutes. Hassle free, fast moving lines, snap snap snap. Then, if you want to meet princesses, you can easily find Alice (Alice in Wonderland) at England, Belle & Beast at Italy, Jasmine at Morroco, Snow White @ Germany, and sometimes even Tinkerbelle and the fairies at some times.

These locations do change, so pick up a map and character times and be early. You need to be in line, BEFORE the character arrives. If you see the character already signing autographs, chances are the line has already closed. It's a 50/50 shot whether or not the line is still open. Usually once people see the character walk out, the line fills up instantly and is closed moments later. They only stay out for about 15 minutes at a time, then they leave for an hour and come back.

Disney World is a bit more scarce. Disney is almost ALWAYS more crowded than Epcot, so finding characters is pretty tough at Disney.

Keep in mind, you are on a time crunch, so the best place to get the characters is always going to be Epcot. Hit character spot, then see if you happen to run into a few more out in the world showcase and call it a day.

To some extent, doesn't this depend on the characters? I was under the impression that the Cars characters were only found in Hollywood studios.

That said, I completely agree with the ease in finding main characters in Epcot. We've had our best success finding Disney Fairies there, usually in the benches/areas to either side just before you get to the lake (just past the test track or imagination). The Aristocats show up in France.

If you're planning well ahead, though - the best way to get time with characters is to schedule character dinners. That's also the easiest way to find characters without necessarily having to enter a park since many of the character dinners are at the Hotel locations. We found Mary Poppins and Alice in Wonderland characters at the Grand Floridian as well as Lilo and Stitch characters at the aptly named Ohana in the Polynesian.

Last thing, are you using Disney, Disney World, and Magic Kingdom interchangeably?